Method of rectification



Nov. 3, 1936.

R. N. SHIRAS METHOD OF RECTIFICATION Filed July 25, 1935 SSM Patented Nov. 3, 1.936

UNITED STATES arsrrr METHOD or nEorurioArroN Russell Norman Shiras, Compton, Calif. Application July 25,1935, serial No. 33,103

2 claims. (o1. 19e-8) This invention deals with the fractionation of natio-n of steps 1 and 3 and my invention comso-called wet gases into normally gaseous and prises an improved and novel process in which normally liquid components. Specifically, it is absorption and stabilization are combined in a concerned with a novel method of producing natsingle operation followed by distillation.

5 ural reflux in the fractionating column of a dis- The equipment which I employ to carry out Ei tillatio-n system in which the top product is a my invention is of the type used for fractional normally non-condensable gas. distillation. It is well known that in a fractional It is ank object of this invention to provide distilling operation a sharp separation can be means, whereby liquerlable components of wet obtained only, if, among other requisites, a sufy ,l l0 gases can be separated economically and sharply cient amount of natural reflux is returned to the le from normally gaseous components. As applied top of the iractionating column, said reflux, as to hydrocarbon gases it is the object of the inventhe name natural indicates, being produced by tion to enable the production of stabilized recova total or partial condensation of the top prodery gasolines in substantially theoretical yields uct. This requirement should be satisfied, even in a manner which is more simple and inexpenif the fractionating column is designed for most sive than has been possible heretofore. efficient operation. In the separation of liquids Wet gases are gases containing components from gases, the top product of the fractionating which are liqueable under normal temperature column is a gas, which may be of such a compoand pressure conditions. The amount of liquesition to make its liquefaction for the purpose fiable material which can be held in solution of of producing a liquid reflux impractical. In some a gas depends upon the temperature and pressure instances the process may be carried out under 0f the gas and` the fugaciily 0f the llqueable` sufficient superatmospheric pressure to enable mailelial- The laWS gOVelIlilg the TeciDfOCal S01- the liquefaction of the normally gaseous top produbilities of vapors and gases are generally known uct, but such methods, if practical, areY usually and require no discussion.` expensive with regard to both equipment and 2'5 The problem of economically recovering liqoperation. uids dissolved in gases is most urgent in the petro- In my novel method I produce a natural reflux leumY industry, the huge quantities of natural and from a gaseous top product without having to cracked gases which are produced daily containresort to excessively high pressures, by contacting ing millions of gallons of recoverable gasoline. the top product with an absorption oil in a cool- 3o In other chemical industries the problem exists ing zone in a manner to Ysaturate the absorption alSO, though t0 a leSSeI degree. Valuable prodoil with gaseous components. The fat o-il prollctS are frequently carried away by Waste 01 duced thereby is used as reflux, and the gaseous other gases and could be recovered, if economical components. dissolved therein play the part; of

means were available. While in the followingY the natural reflux. 35 diScuSSOrl I am mostly concerned With the proc- For instance, top gases and absorption oil are eSSins 0f Wet hydrocarbon gases, it shall be unconducted preferably in concurrent new through dcrStOOd that I d0 not intend t0 limit myself a water cooled condenser of conventional design. tlle'eic7 my iIlVeIllJiOIl being applicable t0 any Concurrent ilow of gas and absorption oil is pre- 40 ses containing in solution Substantial amounts ferred over countercurrent flow because the dan- 4o of liqueiiable components. ger of channeling is eliminated and because of Of the mcJhOdS developed 150 date fOr the abOVe better cooling efiiciency due to a more favorable ifylie Of Separation the DlcceSS ilVclVing the apheat distribution in the condenser. As the two plication of a liquid absorption medium or menuids meet, heat of absorption is developed restruum has proven the most eilective and ilexible. sulting in a temperature rise which may be of 45 Until recently the absorption process hadv to be the order of 10-40 F. The rise in temperature carried out in three separate steps, namely; varies with the composition of the gas and the (1) Absorption of liquefiable components of the absorption oil and with the pressure, at higher gas; pressures more heat being developed. This heat 5o (2) separation of absorbedcomponents from is immediately carried away in thecondenser, re- 50 absorbing menstruurn, viz, distillation; sulting in the absorption of an additional quan- (3) removal of normally gaseous components tity of gaseousrhydrocarbons which may be of from the products recovered under 2, viz, stabilithe order of 20 to 50% of the amount which is zation. absorbable before cooling. The amount of heat .55 Recent developments have enabled the combi developed under these conditions indicates that 55 an actual condensation of normally non-condensable hydrocarbons takes place.

Normally the top gases consist of a mixture comprising methane, ethane, propane, butanes and their corresponding oleines, although olenes and occasionally certain of the other normally gaseous hydrocarbons may be absent. Since the quantity of an individual hydrocarbon which is absorbed is governed not only by its partial pressure but also by its fugacity, the absorbed gas usually does not have the same quantitative composition as the gas which is brought in contact with the absorption oil. The heavier hydrocarbons of lower fugacity are absorbed in preference to the light ones, and consequently the absorbed gas is heavier than the overhead gas. The theoretical requirements characterizing a normal natural reflux, however, are met nevertheless, the resulting natural reflux effecting a satisfactory fractionation.

After passage through the condenser, unabsorbed dry gas and absorption oil are separated in a trap, dry gas is withdrawn and absorption oil substantially saturated with normally gaseous hydrocarbons enters a fractionating column at its top plate and flows in countercurrent to ascending gases, stripping them of normally liquid hydrocarbons. During the descent an exchange of absorbed hydrocarbons takes place, light ones being displaced by heavier ones in a manner similar to the exchange taking place during an ordinary fractional distillation in the presence of normal reflux but in the absence of a third heavier component. The normally gaseous hydrocarbons, absorbed by the absorption oil in the condenser, then behave exactly like a normal reflux, vaporizing while vaporous heavier hydrocarbons are condensed. The absorption oil, acting merely as a carrier, can be considered part of the reilux, and for this reason the saturated absorption oil entering at the top plate of the column is spoken of as total reflux.

'Ihe wet feed gas enters the column at an intermediate point between top and bottom. As the fat absorption oil travels downward and passes this point it contains practically all of the liqueable gas components as well as some normally gaseous ones. To remove the latter, the rich absorption oil is conducted through a stabilizing zone, at the bottom of which there is a reboiler capable of supplying enough heat to vaporize all of the normally gaseous and some of the normally liquid hydrocarbons. The vapors produced in this manner are allowed to rise in countercurrent to fat absorption oil, gradually stabilizing the latter on its way down, and to join the wet feed gases near their point of entrance. The fat absorption oil, when reaching the bottom of the heating zone, is fully stabilized and is withdrawn from the column to be fractionally distilled for the purpose of stripping and recovering absorbed normally liquid hydrocarbons therefrom. The stripped absorption oil, preferably after cooling, is returned to the condenser to repeat the described cycle.

It is obvious that there are a numberof fundamental diilerences between what happens in the zones of contact of feed gas `and absorption oil in a conventional absorber and what takes place in the tower of my system. In the conventional absorber there is a rise in the temperature to a point above the mean temperature of the oil and the gas. The highest temperature is often found in the upper part of the absorption zone, not far down from the point of entrance of the absorption oil or near the point of entrance of the feed gas. Stabilization of the bottom product is therefore not feasible, since an increase in the temperature at the bottom would result in a rise in the temperature near the top, which would either cause the loss of liqueilable components of the gas or require a much higher ratio of oil to gas to prevent such a loss. In contrast to the above, in my column usually there is a gradual decline in the temperature from bottom to top. The amount of the dissolved gas in the absorption menstruum remains substantially the same throughout the absorber, whereas its composition changes due to absorption of less volatile gas components and displacement of more volatile gas components. Stabilization becomes feasible, because a rise in the bottom temperature can be counteracted by a reasonably small increase of the natural reflux, which by its partial vaporization keeps the top temperature at its low level.

In the following example the relative eillciency of my process is compared with that of a conven tional absorber, in which lean absorption oil is fed to the top plate. A gas from a Dubbs cracking unit containing about 2% pentanes and heavier was so processed to recover 99% thereof. The following table indicates the relative proportions of lighter than pentane hydrocarbons which were recovered in conjunction with the pentane:

Percent recovered It is to be noted in the above comparative example that the gasoline produced by my process is well stabilized, while that from the convenitional absorber is very wild.

It is obvious that for the purpose of effective absorption and stabilization a quantity of natural reflux above a certain minimum must be supplied. This minimum can be estimated if the compositions of the wet gas and of the absorption oil are known. The column and the condenser should be operated under such temperature and pressure conditions, to enable the production of a sufficient amount of natural reflux with a minimum amount of absorption oil, high pressure and low temperature at the condenser outlet resulting in a high ratio of natural to total reflux.

The pressures which I apply are usually substantially equal to the pressures of the wet feed gases at their source, and although generally super-atmospheric pressures are of advantage, the cost of compression to very high pressures may not be warranted.

The circulation of a minimum amount of absorption oil is desirable to minimize pumping and distillation costs.

Below is an example of how to estimate the required natural as well as total refluxes.

It is evident that, for taking full advantage of this invention, the latent heat of vaporization of the natural reflux must be at least as great as the latent heat of vaporization of the condensable components of the wet feed gas. Assuming equal molal heats of vaporization for the various components in the gas, it follows that the minimum molal quantity of natural reflux which is required, is equal to the molal quantity of condensable components in the feed gas. This minimum is sufiicient to condense said condensable components but is insufcient to have any effect on the stabilizing zone, an excess over this minimum being required for stabilization.

Thus it is seen that the minimum of the required natural reux equals the wet gas feed, F, minus the dry overhead gas, D. If we assume that the composition of the gas be such that one mol. of dry gas, D, is produced from 1.7 mol. of feed gas, F, and that the pressure and temperature conditions at the condenser outlet, as well as the composition of the dry gas be such, that the fat ab sorption oil comprises 52 mol. percent of absorbed components, we have the following conditions:

Natural reflux, N=.52 total reux, R,

N must be greater than F-D .52 R. must be greater than .7 mol./mol. D.

R must be greater than 1.35 mol/mol. D

This means that the ratio of total reflux to withdrawn product, i. e., reflux ratio, must be greater than 135:1, in order to have an excess reflux available for the stabilizing zone.

A considerable saving of reflux can be achieved by partly condensing` the wet gas prior to admittance to the column. If, for instance, in a condenser situated between the source of the feed gas and the column, 20% of the gas, such as in the above mentioned example, is condensed, .34 mols of liquid is eliminated for every mol. of dry gas and the remaining wet gas amounts to 1.36 mols per mol. of dry gas. Repeating the above calculations, it isfound that the reflux ratio must be greater than .6911. Thus only half the amount of absorption oil needbe circulated as compared with that of the first example.

Further'advantages of the invention are ap- Y parent from the following description of the attached drawing, representingV a iiow diagram of the new process.

Wet feed gas from a source not shown is conducted through line l, condenser Z or by-pass 3, into column 4. The point of entrance 5 of the feed gas is intermediate between top and bottom of the column. The gas rises through the bubble plates 6 which may be of any efficient conventional design and top gas leaves through vapor line l. At point 8 of line l lean absorption oil is admixed to the gas. The resulting mixture is conducted through a water-cooled condenser 9 and through gas separator lll, Oil with the gas absorbed at the temperature of condenser il is withdrawn by way of a conical bottom Il of separator i9 and proceeds through a gooseneclr I2 to the top plate i3 of column ll. Dry gas from the separator leaves through exit line l-l. As the saturated absorption oil descends over the bubble plates 6 in countercurrent to the rising gas, lighter hydrocarbons dissolved in the absorption oil are exchanged for heavier ones. The zone in column 4 below the point of entrance 5 of the feed gas, represents a stabilizing zone. Heat is provided by reboiler i5 located near the bottom of the stabilizing zone, said heat causing the evaporation of the lighter absorbed hydrocarbons. Stabilized fat absorption oil is drained through bottom line i6 and valve l1 to be stripped in a separate distillation unit, comprising a column i8 with heating coil I9 near its bottom, Vapor line 2B and condenser 2| for overhead products, which after leaving the condenser go to storage as recovery gasoline. Steam may be in troduced through line 26 to facilitate the stripping operation. The bottom product proceeds through line 22 or cooler 23 to be picked up by pump 2li and returned to point 8 in vapor line l of the absorption system.

1t is understood that the equipment asdescribed does not represent the only type of apparatus in which my invention can be carried out. For instance the absorption and stabilizing section may be embodied in separate structural units; or the condenser and separator may be a structural part of the absorbing and condensing unit. Furthermore, the condenser can be such that absorption oil and gas meet in countercurrent rather than concurrent ilow although admittedly the method substantially as shown is the Apreferred one.

I claim as my invention:

l. In the continuous ebullient fractionation of vapors containing normally gaseous and liqueable components to produce a normally gaseous overhead and a liquid bottom product, the method of providing natural reux comprising ilowing the overhead gas and an absorption menstruum for the said vapors concurrently through an externally cooled zone simultaneously to absorb a portion of the gas in said menstruum and to withdraw heat of absorption from the menstruum, separating unabsorbed gas from the cooled menstruum and flowing the cooled men- Vstruum containing absorbed gas oountercurrently against said vapors through a fractionating zone.

2. In the continuous ebullient fractionation of vapors containing normally gaseous and liqueable components to produce a normally gaseous overhead and a liquid bottom product the improvement comprising flowing the overhead gas and an absorption menstruum for said vapors concurrently through an externally cooled zone simultaneously to absorb a portion of the gas in said menstruum; and to withdraw heat of absorption from the menstruum, separating the unabsorbed gas from the cooled menstruum, flowing the cooled menstruum containing absorbed gas in countercurrent to said vapors in a fractionating zone under conditions to produce the overhead gas substantially free of liquefiable components and a fat menstruum containing substantially all of the liqueiiable components and a portion of the normally gaseous components of said vapors, subjecting the fat menstruum to fractional distillation under conditions to-separate therefrom substantially only the excessively volatile components, and combining said separated components with the vapors to be treated.

RUSSELL NORMAN SI-HRAS. 

